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Allg. 11, 1959 R H, W|N`KLER 2,899,595

RESONANT WAVEGUIDE BACKWARD-WAVE OSCILLATOR Afro/.wigs

l Ang. 11, 1959 R, H wlNKLER 2,899,595

RESONANT WAVEGUIDE BACKWARD-WAVE OSCILLATOR Filed Oct. 4. 1957 2 Sheets-Sheet 2 INVENTOR 'Pw/mea M dm/ui? Lil/W Arroz/vir;

microwave modes of a type commonly associated with coaxial transmission lines. A noteworthy characteristic of such modes, of importance to the structure illustrated, is that such coaxial wave-transmission structures have no low-frequency cutoff. For example, if a D.C. electric source were connected between plate 11 and envelope 1 at the'open-circuited end of the resonator, direct current would be transmitted to the shorted end of the structure without any losses except those due to conductor resistance.

, On the other hand, there are significant differences be- 'tween the structure shown in Figs. 1 and 2 and a short section of an ordinary coaxial transmission line. In particular, the interdigitated structure that defines serpentine passage V17 supplies a heavy capacitive loading to the line and concentrates the microwave electric field within and 'near the serpentine passage. Such a structure does have a high-frequency cutoff corresponding to a wavelength atV which the length of each tooth (in a direction transverse to the longitudinal axisV of cylindrical envelope 1) is approximately equal to one-half wavelength.

The resonant frequencies of the resonator are those frequencies that lie between the low-frequency cutoff (zero frequency in this case) and the high-frequency cutoff, and that also correspond to wavelengths at which the serpentine length of passage 17 is approximately equal to an odd integral number of quarter wavelengths. In lthe structure illustrated in Figs. 1 and 2 it is evident that there are four such resonant frequencies. The lowest resonant frequency is the one corresponding to a wavelength approximately four times as long as the serpentine length of passage 17.' The other resonant frequencies are the third, fifth and seventh harmonics of the lowest resonant frequency. The oscillator illustrated in Figs. 1 and 2 is essentially a fixed-frequency device, operable approximately at one of these four resonant frequencies only. From the foregoingconsiderations, it is evident that the structure illustrated in Figs. l and 2 has certain characteristics of a low-pass filter. It is well known that numerous space harmonics are associated with electromagnetic waves transmitted along filter-like circuits. The present structure is no exception. In general, these space harmonics have phase velocities that are very small compared to the velocity of light, and therefore are capable of being approximately matched by easily produced electron velocities for producing energy-transferring interactions between the electromagnetic waves and a moving stream of electrons. In the resonator structure under consideration, the microwaves travel back and forth repetitively through serpentine passage 17,and therefore there are both forward-wave and backward-wave space harmonics having phase velocities traveling in the same direction as the electron beam.

In principle, the electron velocity could be approximately matched to the phase velocity of any one (o1 sometimes more) of the space harmonics to obtain an energy-exchanging relationship between the electron beam and the electromagnetic Wave. In practice, with the short structures used in -practicing this invention, interactions usually occur only with the first (fastest phase velocity) backward-wave component. The reason is that this space harmonic has the highest coupling impedance to the electron beam, and therefore requires the lowest beam current to establish and to sustain microwave oscillations. The higher-order space harmonics in particular have much lower coupling impedances to the beam, and consequently are much less likely to interact suiciently for the production of sustained microwave oscillations.

In some cases, particularly when operating at the lowest resonant frequency of the resonator, appreciable interactions. may occur with both backward-wave and forwardwave space harmonics of the lowest order. However, no disadvantageous results follow from this, because the resonator length restricts the build-up of oscillations to certain fixed, harmonically related frequencies, as herezi inbefore explained, and the frequencies of first-order forward-wave and backward-wave interaction are sutiiciently close that oscillations will usually occur at only one of these fixed frequencies for a given beam voltage.

Like other filter-type circuits, the resonator disclosed in Figs. 1 and 2 is dispersive. That is, the phase velocity of each space harmonic varies considerably as a function of frequency. Since energy-exchanging interactions occur only when the electron velocity is approximately equal to the phase velocity of a space harmonic, the beam voltage determines which of the several resonant frequencies of the resonator will be taken by the oscillations that are set up. This can be controlled inthe usual manner by varying the potential of the cathode relative to the potential of envelope 1-and partsconnected thereto.

For example, a relatively small beam voltage produces a correspondingly small electron velocity, which may set upi-nthe resonator oscillations at the lowest resonant frequency correspondingV to a'wavelength approximately four times as long as the serpentine length of passage 17. A somewhat higher beam voltage may set up oscillations at the third harmonic frequency, at which serpentine passage 17 is approximately three-quarters of a wavelength long. A still higher beam voltage may excite oscillations at the fifth harmonic frequency, etc. The tube is generally operated at frequencies such that the length of passage 17 is less than one wavelength.

Another distinction, between the present tube and a conventional backward-wave oscillator may be Vnoted at this point. In backward-wave oscillators, electromagnetic wave energy is transmitted in a direction counter to the electron flow. In conventional tubes, wave reflections are minimized with attenuators, and the major energy ow is in this one direction. Furthermore,a fairly long (many wavelengths) slow-wave structure is used, which usually has a substantially constant impedance along its length. Consequently, the microwave electric tield associated with the generated microwaves is strongest at that end of the slow-wave structure nearest the electron gun, and is relatively weak at the collector end of the slow-wave structure.

In the tube'illustrated in the Figs. 1 and 2, conditions are quite different. As in a conventional travelingwave tube, Velectromagnetic waves traveling from right to left through passage 17 (as viewed in Fig. l) gain energy by backward-wave interaction with the electron beam. However, the electromagnetic waves are reected at both ends of passage 17, and travel repetitively back and forth through the serpentine wave-transmitting passage. Consequently, the magnitude of the electromagnetic wave energy is substantially constant throughout the length of the slow-wave structure. However, the microwave electric field, which interacts with the electron beam, is not constant.

Assume, for example, that the oscillation frequency corresponds to the lowest resonant frequency of the resonator, at which passage 17 is approximately onequarter wavelength long. As is well known, a quarterwave resonant structure exhibits a standing-wave pattern having a voltage minimum at the short-circuited end and a voltage maximum at the open-circuited end of the resonator. Consequently, in the structure illustrated, the microwave electric field, which interacts with the electron beam, is -weaker where the electrons first enter the field, and stronger at the collector end of the structure. This is just the reverse of the situation in conventional backward-wave oscillators. An' important end result 'is a higher electrical efficiency in the new tubes.

This can be understood from the following considerations. Near the point where the electron beam first enters the microwave field, the chief effect of the interaction between the electric field and the beam is a velocity modulation of the beam, which absorbs energy from the microwave field. The stronger the microwave elecim velocity, ,the .greater the amsunt pien' that frequency 'oor'r'e'spording to a Waveleri'g'tli of approgii mately twice the serpentineulength' of passage 29, T5 avoid losirigthe lowest resonant frequericy, the da'reter vcylindrical envelope 18 preferably israde suircie "ly large that the low'ffr'e'qency'cutotf falls beljoyy the equericy at which passage 29 is 'onehalf 4v v ayelefrigtl'i l The lop"efratiig p'ririciples fithis tube 'areye'lry Isi P ig. V3), there are twofcdctive Ietal plates 27 I iKelytobe rriodulate'dothe extentth 't'thebe n iay begin absorbing energy from rather tlijai'i up'ply g t me the evelolie a rui Verse section ofthis resonator is sho Ffthrmore, irl practice tli electron beam usually Wellffocus'ed `oly' to abo 1 1t tl `1e center of 'the res'n r, sice rio ma'getic iel'd lor other focusing eahs the other plate. Thus, the 'toothed edlgfs L )loj/ q b eyorid the electron gu n As 'die beau@ beo .and 28 cooperate to form an, open-sided, .ds-tfswused ,and Spreads, aprtion Qf 111i@ beahygis i tajsmifrig passage 429, Each ,frhe iper ses@ by: sashf. th imerdisitel treatment-in@ llas au aperture 22aliridjviftl1 aperture ZZto prrri tl 1 e uen e. the beam current decreases substatially towar lgassage yof electron 2 6 t'hroughtlietlee'th It 1s api the collector. erid of the resonator. .i

agitated wth but seagate@ frati-1.1116. 'to -f a 'fem that the eieos repeufiveiy foss the serpentine; The embddimem iihi'streiiin rig. 3jd` a iv'aye-trasiriittirig passage 29, 70 what longer highfrppedance interaction regiori t oes @what the @embodiment iuusfrad in Fig. .1, aadaisg ae ear e fo the ether een;

dept illustrated iii 3 isvcapablerof opleratio `A3 1 odd `arid 'evenharmoriicsot the lowest 'r` I ant fr The@ .edd tooth lngtl;A rel A maar tran mi isn une h -7 tion, the embodiment illustrated in Fig. 3 is somewhat larger than the embodiment illustrated in Fig. l. These differences may be advantages or disadvantages of one embodiment relative to the other, depending upon the particular environment and circumstances in which the tubes are to be used.

Still another example of a backward-wave oscillator tube incorporating certain principles of this invention is illustrated in Figs. 5 and 6. An evacuated, hollow, right circular cylindrical, electrically conductive, metal envelope 35 is closed at one end by a metal end plate 36 and is closed at the other end by an insulating base member 37. The space within the envelope is divided into two lengthwise sections by an electrically conductive, metal, transverse partition 38 having a small circular aperture 39 through the center, as shown.

To the left of partition 38 (as viewed in Figs. 5 and 6) there is aconventional electron gun comprising a heater filament 40, a cathode 41 and a focusing electrode 42. Electrical supply leads for the electron gun extend through insulating base member 37 in the usual manner, as shown. The electron gun provides a focused beam of electrons, represented in the drawing by broken line 43, that pass through aperture 39 and move from left to right along the longitudinal axis of cylindrical envelope 35. lartition 38 acts as a first anode or accelerating electrode of the electron gun. The electrons are collected by end plate 36, which acts as a collector electrode for the electron beam,

Within envelope 35 to the right of partition 38 (as viewed in Figs. 5 and 6) there are two electrically conductive, metal plates 44 and 45 disposed side-by-side in a common diametrical plane of cylindrical envelope 35. Each of the Iplates 44 and 45 has a toothed edge interdigitated with but separated from the toothed edge of the other plate, as shown. For example, plate 44 may be provided with three projecting teeth 46, 47 and 48 that are interdigitated with three teeth 49, 50 and 51 projecting from plate 45. The toothed edges of plates 44 and 45 cooperate to define a serpentine, wave-transmitting passage 52.

The right end of plate 44 is connected to the conductive cylindrical envelope 35, while the left end of plate 45 is connected to envelope 35. Furthermore, tooth 46 of plate 44 is connected to envelope 35 through a conductive bar 53, and tooth 51 of plate 45 is connected to envelope 35 through a conductive bar 54, for purposes hereinafter explained. Otherwise, plates 44 and 45 are separated from envelope 35 so that, at microwave frequencies, major portions of the lengths of plates 44 and 45 are insulated from envelope 35. Thus, the structure is somewhat analogous to a shielded-pair transmission line, and is capable of supporting electromagnetic waves in microwave modes of a type commonly associated with shieldedpair transmission lines.

The structure illustrated in Figs. 5 and 6 differs from a short section of an ordinary shielded-pair line in that the interdigitated teeth of plates 44 and 45 provide capacitive loading and form a filter-type slow-wave structure capable of supporting electromagnetic waves with space harmonies that have small phase velocities compared to the velocity of light. Furthermore, the structure is electrically short, and is terminated at each end by nondissipative wave-reflecting terminations to form a microwave resonator.

The two conductive bars 53 and 54 are bent into a somewhat S-shaped configuration, as shown, and have outer ends that are connected to conductive cylindrical envelope 35. Bars 53 and 54 act as short stub transmission lines and provide nondissipative reactive terminations at each end of the serpentine, wave-transmitting passage defined by the interdigitated structure. Assume, for example, that the serpentine passage 52 is approximately one-quarter wavelength long at a particular operating frequency, and that each of the bars 53 and 54 forms 4a stub-transmission line approximately one-eighth wavelengthI long. It is evident that the structure as a whole is approximately one-half wavelength long, and forms a resonator lwherein electromagnetic waves are multiply reflected and travel repetitively back and forth through serpentine passage 52.` Because passage 52 is a quarterwave section located approximately at the electrical center of a half wave resonator, the microwave electric field has an appreciable amplitude throughout the length of Vserpentine passage 52. Consequently, there is a fairly high coupling impedance between the electromagnetic waves and the electron beam throughout the length of passage 52.

The two bars 53 and 54 need not be precisely the same length. By making bar 53 somewhat shorter than bar 54, as shown in Fig. 6, the microwave electric field can be made somewhat weaker at the end of passage 52 nearest the electron gun than it is at the other end of the passage, which is desirable in accordance with principles hereinbefore disclosed. Thus, bar 53 is a means for providing a microwave electric field that is relatively small, but not zero, at the leftend of passage 52, while bar 54 provides a terminating means at the right end of passage 52 such that the structure as a whole is a half-wave resonator. It is evident that the structure will also resonate at higher frequencies corresponding to wavelengths at which the structure as a whole has an electrical length approximately equal to an integral multiple of a half wavelength. Therefore, the structure has a plurality of hairnonically related resonant frequencies, lying between a lower cutoff frequency and a higher cutoff frequency, similar to the other resonators herein described.

The electromagnetic waves supported in the resonator travel through serpentine passage 52 and there produce space harmonics having phase velocities that are small compared to the velocity of light, in the usual manner of interdigital wave-transmission structures. When the electron velocity of beam 43 is made approximately equal to the phase velocity of one of these space harmonics, energy-transfering interactions occur between the electromagnetic waves and the electron beam so that, when the electron velocity is slightly greater than the phase velocity, energy is transferred from the electron beam to the electromagnetic waves and sustained oscillations are produced approximately at a resonant frequency of the resonator.

Coupling means for withdrawing electromagnetic microwave energy from the resonator may, for example, comprise a coaxial transmission line having an outer conductor 55 connected to conductive end plate 36 of the evacuated envelope and an inner conductor 56 that extends through an iris 57 in the end plate and is bent into an inductive coupling loop 58. Microwave current passing through bar 54 and other parts of the resonator structure produces a microwave magnetic field that links the coupling loop 58 and supplies microwave energy to the output transmission line by ordinary transformer action. An insulator 59 within the transmission line serves as a vacuum seal for maintaining the vacuum within envelope 35.

It should be understood that this invention in its broader aspects is not limited to specific examples herein illustrated and described, and that the following claims are intended to cover all changes and modifications within the true spirit and scope of the invention.

What is claimed is:

l. A backward-wave oscillator comprising the following combination: an evacuated microwave resonator containing a serpentine wave-transmitting passage, said passage having non-dissipative wave-reflecting terminations at both of its ends so that electromagnetic waves within said resonator are repetitively reflected at said terminations and travel back and forth through said passage, whereby there are produced backward-wave space harmonies having phase velocities that are small compared to the velocity of light; means for providing a beam of electrons moving along a path repetitively crossing said serpentine passage at a velocity approximately equal to the phase velocity of one of said backward-wave space harmonies, so that said beam supplies energy to said electromagnetic waves, whereby sustained electromagnetic oscillations are generated at a resonant frequency of said resonator; and coupling means for withdrawing microwave electromagnetic energy from said resonator.

2. A backward-wave oscillator as defined in claim 1, wherein the serpentine length of said passage is smaller than one wavelength of said electromagnetic waves at the lowest resonant frequency of said resonator.

3. A backward-wave oscillator as delined in claim 1, wherein said wave-reecting terminations are a microwave short circuit and a microwave open circuit, respectively, said short circuit being at that end of said serpentine passage that is rst crossed by said electrons, and said resonator has a resonant frequency at which the length of said serpentine passage is approximately equal to a quarter wavelength of said electromagnetic waves.

4. A backward-wave oscillator as dened in claim l, wherein both of said wavereecting terminations are microwave short circuits, and said resonator has a resonant frequency at which the length of said serpentine passage is approximately equal to a half wavelength of said electromagnetic waves.

5. A backward-wave oscillator as delined in claim l, wherein said waveJr-eflecting terminations are microwave reactances provided by stub transmission lines.

6. A microwave electron tube comprising the following combination: an evacuated envelope; two coplanar, electrically conductive plates within said envelope, each of said plates having a toothed edge interdigitated with but spaced from the toothed edge of the other plate, defining an open-sided serpentine passage between the two plates for the transmission of electromagnetic waves, said passage having non-dissipative wave-reiiecting terminations at 'both of its ends, so that said waves travel back and forth repetitively through said passage, thus forming a microwave resonator having a plurality of resonant trequencies; and electron gun means within said envelope for providing an electron beam that repetitively crosses said serpentine passage in energy-exchanging interacting relation with said electromagnetic waves.

7. A microwave electron tube comprising the following combination: an electrically conductive, evacuated, hollow cylinder; two electrically conductive plates disposed side-by-side in a common diametric plane Within said cylinder, said plates having interdigitated, toothed, adjacent edges defining `an open-sided serpentine passage for the transmission of electromagnetic waves lwith space harmonies having phase velocities that are small compared to the velocity of light, said passage having non-dissipative wave-reflecting terminations at both of its ends so that said waves travel back and forth repetitively through said passage, thus forming a microwave resonator having a plurality of resonant frequencies; means for providing a beam of electrons moving along a path repetitively crossing said serpentine passage at a velocity approximately equal -to the phase velocity of one of sa-id space harmonics so that `said beam supplies energy to said electromagnetic waves; and `coupling means for withdrawing microwave energy tom said resonator.

8. A microwave electron tube as `defined in claim 7, wherein with `respect to microwaves, one of said plates is electrically connected throughout a major portion of its length to said cylinder lwhile the other of said plates is electrically insulated throughout 4a major portion of its length from said cylinder, so that said resonator supports electromagnetic waves in microwave modes of a type commonly associated with coaxial transmission lines.

9. A microwave electron tube as defined in claim 7, wherein, with respect to microwaves, both of said plates are electrically connected throughout major portions of their lengths to said cylinder, so that said resonator supports electromagnetic waves in microwave modes of a type commonly associated with ridged waveguides.

10. A microwave electron tube as defined in claim 7, wherein, with respect to microwaves, both of said plates are electrically insulated throughout major portions of their lengths from said cylinder, so that said resonator supports electromagnetic waves in microwave modes of a type commonly associated with shielded-pair transmission lines.

1l. An oscillator for supplying electromagnetic waves to a waveguide, comprising an electrically conductive, hollow cylindrical envelope having an open end attached to `and opening int-o the waveguide, two electrically conductive plates disposed side-by-side in a common diametric plane within said cylindrical envelope, one of said plates being electrically and mechanically connected substantially throughout its length to said envelope, the other of said plates being electrically and mechanically separated substantially throughout its length from said envelope, said plates having interdigitated, toothed, adjacent edges defining an open-sided serpentine passage for the transmission of electromagnetic waves, the edges of said plates nearest to the waveguide being electrically separated to form an electrical open circuit at that end of the serpentine passage, a conductive, transverse partition extending across said envelope, the edges of said plates farthest from the waveguide being electrically and mec'hanically connected to said partition to form an electrical short circuit at that end of the serpentine passage, said paitition and a plurality of said teeth containing apertures alined substantially along the longitudinal axis of said cylindrical envelope, and electron gun means disposed vvithin said envelope on the other side of said partition from the waveguide for providing an electron beam through such apertures.

References Cited in the le of this patent UNITED STATES PATENTS 2,817,040 Hull Dec. 17, 1957 2,820,170 Robertson Jan. 14, 1958 2,824,256 Pierce et a1 Feb. 18, 1958 2,827,589 Hines Mar. 18, 1958 

